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MAX9517

Standard-Definition Video Filter Amplifiers with Dual SPST Switches

Maxim Integrated 

Standard-Definition Video Filter Amplifiers with

Dual SPST Switches

Detailed Description

The MAX9517/MAX9524 consist of a lowpass filter and

an output amplifier capable of driving a standard 150Ω

video load to ground. The MAX9517 has an input buffer

and the MAX9524 has an input sync-tip clamp. The

MAX9517/MAX9524 both have two SPST analog

switches that can be used to route audio, video, or digi-

tal signals. The output amplifiers provide a fixed gain of

2V/V.

The MAX9517/MAX9524 filter and amplify the video

DAC output. External video signals, in which the DC

bias is usually not known, can be AC-coupled to the

MAX9524.

Input with DC Buffer (MAX9517)

The input of the MAX9517 can be directly connected to

the video source if the signal is approximately between

ground and 1V. This specification is commonly found at

the output of most video DACs.

DC-coupling requires that the input signals are ground

referenced so that the sync tip of composite or luma

signals is within 50mV of ground.

Input with Sync-Tip Clamp (MAX9524)

When the bias of the incoming video signal is either

unknown or not between ground and 1V (such as an

external video source), use the MAX9524 to connect

the video source through a 0.1µF capacitor.

The VIDIN input of the MAX9524 can only handle sig-

nals with a sync pulse, such as composite video and

luma. An internal sync-tip clamp sets the internal DC

level of the video signal.

Video Filter

The filter passband (±1dB) is typically 9MHz to make

the device suitable for standard-definition video signals

from all sources (including broadcast video and DVD).

Broadcast video signals are channel limited: NTSC sig-

nals have 4.2MHz bandwidth, and PAL signals have

5MHz bandwidth. Video signals from a DVD player,

however, are not channel limited; therefore, the band-

width of DVD video signals can approach the Nyquist

limit of 6.75MHz (recommendation ITU-R BT.601-5

specifies 13.5MHz as the sampling rate for standard-

definition video). Therefore, the maximum bandwidth of

the signal is 6.75MHz. To ease the filtering require-

ments, most modern video systems oversample by two

times, clocking the video current DAC at 27MHz.

Outputs

The video output amplifiers can both source and sink

load current, allowing output loads to be DC- or AC-

coupled. The amplifier output stage needs around

300mV of headroom from either supply rail. The parts

have an internal level shift circuit that positions the sync

tip at approximately 300mV at the output.

If the supply voltage is greater than 3.135V (5% below

a 3.3V supply), each amplifier can drive two DC-cou-

pled video loads to ground. If the supply is less than

3.135V, each amplifier can drive only one DC-coupled

or AC-coupled video load.

Shutdown

The MAX9517/MAX9524 draw less than 1µA supply

current when SHDN is low. In shutdown, the amplifier

output becomes high impedance.

SPST Analog Switches

Table 1. Logic for Analog Switches

IN_

SWITCH STATE

0

OFF

1

ON

Applications Information

Reducing Power Consumption

in the Video DACs

The MAX9517/MAX9524 have high-impedance input

buffers that can work with source resistances as high

as 300Ω. To reduce power dissipation in the video

DACs, the DAC output resistor can be scaled up in

value. The reference resistor that sets the reference

current inside the video DACs must also be similarly

scaled up. For instance, if the output resistor is 37.5Ω,

the DAC must source 26.7mA when the output is 1V. If

the output resistor is increased to 300Ω, the DAC only

needs to source 3.33mA when the output is 1V.

There is parasitic capacitance from the DAC output to

ground. That capacitance in parallel with the DAC out-

put resistor forms a pole that can potentially roll off the

frequency response of the video signal. For example,

300Ω in parallel with 50pF creates a pole at 10.6MHz.

To minimize this capacitance, reduce the area of the

signal trace attached to the DAC output as much as

possible, and place the MAX9517/MAX9524 as close

as possible to the video DAC outputs.

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